for the method, see Eykhoff in trends and progress in system
identification) page 96. with

z(t)=[y(t-1),..,y(t-r),u(t),...,u(t-s),e(t-1),...,e(t-q)]

and

coef=[-a1,..,-ar,b0,...,b_s,d1,...,d_q]'y(t)=coef'*z(t)+sig*e(t).

a sequential version of the AR estimation where e(t-i) is replaced
by an estimated value is used (RLLS). With q=0 this method is exactly
a sequential version of armax

Important notice

In Scilab versions up to 4.1.2 the returned value in
arc.sig is the square of sig
square. To be conform with the help, the display of arma models
and the armax function, starting from Scilab-5.0 version the
returned arc.sig is sig.